Metal-phthalocyanines (MPcs) are a class of organic macrocycles that have received much attention for their potential use in various applications, including molecular electronics. In recent years, the on-surface synthesis of different MPcs has been demonstrated via deposition of atomic species, sample heating and scanning tunneling microscopy (STM) manipulation. However, the spontaneous substitution of the central metal atom with a substrate one, known as transmetalation, remains a less explored process. Here, we study the transmetalation of ZnPc on Al(100) using a combination of STM and density functional theory (DFT) computations. Previously published X-ray photoelectron spectroscopy (XPS) data indicate the absence of Zn atoms from the interface, while our STM images show that a central atom is still present. This suggests that the transmetalation process occurs spontaneously for all ZnPc molecules. Our DFT calculations reveal that the transmetalation of Zn with Al releases 2 eV per molecule, providing thermodynamic evidence for the process. The on-surface synthesized aluminium phthalocyanine (AlPc) molecules are stabilised by the formation of a bond between the central metal atom and the substrate, similar to that found in chloroaluminium phthalocyanine (ClAlPc), with the consequent suppresion of the paramagnetism of free AlPc.
